Mrp-L M-Lok 5.56/300 Blk 13.25'' Assembled Upper Receiver

The LMT Defense MRP-L M-Lok assembled upper receiver in 5.56/300 Blackout is a rifle-length platform built for builders who want a precision, modular foundation without compromise. This 13.25" receiver pairs with 14.5" to 20" barrels and represents LMT's monolithic rail platform philosophy: the receiver and handguard are forged as a single piece of aerospace aluminum, eliminating flex and rattle between components. If you're assembling a duty rifle, precision build, or mid-range hunting platform, this upper gives you the rigidity and modularity serious shooters demand.

What sets the MRP-L apart is its barrel attachment system. Instead of a traditional extension, LMT uses a proprietary locking mechanism with two bolts accessed from the right side of the receiver—tightened with just a standard T-30 Torx bit. This design keeps the receiver cleaner and more streamlined than conventional setups. The integrated monolithic rail runs the full length of the handguard with M-Lok compatible slots on all eight sides, giving you genuine freedom to mount optics, lights, vertical grips, or covers exactly where you need them. Four QD sling attachment points are built in, so you're not hunting for hardpoints or improvising solutions.

This is an assembled upper receiver—the hard part is done. You'll need a LMT monolithic barrel assembly (sold separately) chambered in 5.56 or .300 Blackout, and the barrel must be compatible with the MRP platform. The receiver is not a drop-in; it's a premium piece engineered for shooters who understand the value of integrated design and are willing to follow the LMT system. No barrel, bolt carrier, or charging handle included; this is the foundation.

Key Features

• Monolithic forged aluminum construction (receiver and handguard as one unit) for maximum rigidity and zero play
• Proprietary dual-bolt barrel locking system—tool-free adjustments with only a T-30 Torx bit needed
• M-Lok attachment points on all 8 sides of the handguard for complete modularity
• Four integrated QD sling swivel mounts
• Rifle-length platform rated for 14.5" to 20" barrels
• Accepts .204 Ruger, .300 Blackout, 5.56×45, and 6.8 SPC with LMT monolithic barrel assemblies

Specifications

Frequently Asked Questions

Can I use any 5.56 or .300 Blackout barrel with this upper receiver?

No. The MRP-L requires a LMT monolithic barrel assembly—standard AR-15 barrels and extensions are not compatible. The proprietary barrel locking system is specific to the MRP platform. You must use a barrel designed for LMT MRP receivers.

What do I need to complete this upper receiver?

You'll need a LMT monolithic barrel assembly in your chosen caliber, a bolt carrier group, and a charging handle. The receiver itself is the foundation; those three components are the essential additions to make it function.

What's the advantage of the monolithic design?

By forging the receiver and handguard as a single piece, LMT eliminates any flex or play between components. This rigidity improves accuracy, optics performance, and overall reliability—especially valuable in precision or duty applications. It also reduces the number of fasteners and potential failure points.

How many attachment slots does the handguard have?

The 13.25" handguard includes M-Lok compatible slots on all eight sides (top, bottom, and both sides), giving you complete flexibility to mount optics, lights, grips, and other accessories without hunting for limited hardpoints.